The differentiation of lens cells in vivo involves the transition of dividing epithelial cells to nondividing fiber cells containing the lens specific protein gamma-crystallin. To determine conditions in vitro which modulate lens cell division and differentiation, lens primary cultures were prepared from rat embryos or 1-5 day newborn rats in Medium 199 containing 20 percent fetal bovine serum (FBS). After attachment, cells underwent the following morphological alterations: elongated yields polygonal elongated yields epithelial-like with eventual nuclear pyknosis. Although the rate of morphological change was independent of FBS concentration, cell proliferation was dependent upon the FBS concentration between 2.5 and 20 percent. Cells cultured in medium containing 1.25 percent FBS did not increase in number whereas cells cultured in 0.6 or 0.3 percent FBS did not survive. Addition of 10 to the minus 7th power-10 to the minus 4th power M thymidine to 1.25 percent FBS medium stimulated cells to multiply at a rate similar to 20 percent FBS grown cells. Cells cultured 3 weeks in medium containing 20 percent FBS synthesized gamma-crystallin. Studies to determine if thymidine addition to low serum containing medium stimulates gamma-crystallin synthesis are in progress. In conclusion, these results indicate that medium 199 containing 20 percent FBS supports lens cell division and differentiation and that the high FBS requirement for growth of these cells can be replaced by addition of thymidine to the culture medium.